Understanding the strategies utilized by a host to defend against infections and reduce disease severity is one of the ultimate goals of immunology and studies of host-microbe interactions. Hosts have evolved two types of defense strategies to limit the impact of infection on health. First, hosts can use resistance mechanisms to directly attack a pathogen to block invasion or eliminate the infection. The second strategy includes tolerance mechanisms that work to limit the health costs to a host by minimizing the amount of damage caused by a given number of microbes. Typically in our immunological studies, we focus on how a host kills pathogens. By contrast, studies that focus on tolerance are limited and therefore the molecular mechanisms of tolerance are far less well understood. This project is designed to utilize the interactions between a host and commensal bacteria, specifically the intestine and the commensal flora, as a model system to identify novel tolerance mechanisms that may be applied to pathogenic infections. The human intestine harbors trillions of bacteria that are involved in an active dialogue with the mucosal immune system. For the majority of hosts, these interactions occur with no adverse effects on health. However, dysregulation of these mechanisms may cause a reduction of tolerance leading to severe pathologies such as Crohn's disease (CD). Strong evidence suggests that the commensal bacteria appear to be essential for the development of CD and the bacterial protein, flagellin, has been found to be a dominant antigen in CD patients. Therefore, the intestine and the commensal flora provide an exciting opportunity and ideal model system to discover novel tolerance mechanisms. The goal of this project is to elucidate an innate immune mechanism that mediates flagellin-exacerbation of inflammatory bowel disease using an in vivo mouse model of colitis, mouse genetics and conventional immunological and microbiological techniques. In preliminary experiments, mouse mutants deficient for the function of two flagellin-sensing proteins of the innate immune response, Naip5 and Ipaf, were found to be resistant to colitis compared to wild type mice. The proposed experiments are designed to test the hypothesis that Ipaf- and/or Naip5-mediated responses to flagellin are important in the development and progression of inflammatory bowel disease. The specific aims of the project are: (1) Determine the role of Naip5 and Ipaf function in the progression of inflammatory bowel disease; (2) Determine how Naip5 and Ipaf affect the microbiota in the presence and absence of gastrointestinal insult; and, (3) Determine the role of commensal microflora recognition in Naip5/Ipaf-mediated exacerbation of gastrointestinal inflammation. The proposed experiments will provide insight into how hosts can tolerate constant exposure to a commensal-derived ligand and provide a molecular mechanism that may be involved in the progression of IBD.